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OK. Let's get started.
We're going to complete our
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discussion about mitosis
and meiosis today.
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And then get into a topic called
Mendelian Genetics,
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understanding genetic principles.
And we'll give you as examples the
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work of the famous Austrian monk
Gregor Mendel who sort of set down
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the rules that we still follow today
regarding simple genetic inheritance.
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So I mentioned to you last time,
and I think you all know that your
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genes are carried on chromosomes.
And chromosomes come in different
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sizes and shapes.
You have 46 total chromosomes in
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all of your cells.
Those chromosomes come from your
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parents. You get one set from one
parent and another set from another
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parent, so you have 22 pairs plus
the two sex chromosomes.
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And the chromosomes are made of DNA,
and along those DNA sequences are
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your genes scattered along the
length of the chromosomes.
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And there are about 30,000 genes in
total.
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Now, I've also told you that the
genome, the whole sequence of your
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DNA has been determined over the
last couple of years.
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We now know that in great detail.
We know every single nucleotide of
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the three times ten to the ninth
base pairs of DNA,
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and we can actually go to a website
and look at it.
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You can do this.
It's publicly available,
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sponsored by the government,
but it's a little like hacking into
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the database of humanity.
So if you go to this website that I
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just showed you,
you'll find access to the genomes of
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many organisms.
This is an evolutionary tree which
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shows the relationship of these
organisms from an evolutionary
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perspective. There are nine
mammalian genomes that are present
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within this database.
If you clink on that link to
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homosapiens, that's us,
assuming this is functioning.
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I had this working.
I'm not sure why it's not working.
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I'm getting a good signal here. Oh
well. Had this worked,
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you would have seen an alignment,
a diagram of all the human
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chromosomes, one through 22,
plus X plus Y. You could then click
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on chromosome whatever you're
interested, let's say chromosome one.
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It would show a diagram of
chromosome one from end to end.
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And you could then click on the
sequence of chromosome one.
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All two point four times ten to the
eighth base pairs of chromosome one.
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It would show you all of the genes
on chromosomes one.
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You would know exactly where they
were. And you could then click on a
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particular gene and look at its
sequence. And it would actually
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give you information about that gene,
as much information as is known.
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So, we know the nucleotide sequence
of all the genes.
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And the question is,
is that sequence that's in that
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database applicable to all of you?
Is that the sequence of your DNA?
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What do you say? Why do you say no?
Right. So it's somebody's DNA.
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Actually, the genome sequence
that's present in a publicly
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available database is a collection
of a small number of people's DNA,
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but it's a representation of the
human genome. And you all have
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subtle differences.
In between genes quite a few,
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and within your genes some. And we
call those differences within genes
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allelic differences,
you have different alleles,
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and that's what makes you all
different, which alleles you carry,
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and importantly which combinations
of alleles of genes that you carry.
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OK. I apologize for that not
working. I don't know why it didn't.
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All right. Well,
last time we left off talking about
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the fact that you inherit your
chromosomes through the process
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of fertilization.
Fusion of sperm and egg produces a
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diploid cell from two haploid cells.
This diploid cell has two copies of
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all the chromosomes,
two copies of all the genes,
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and then through the process of
development there is a great deal of
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mitosis. We talked about mitosis in
some detail last time.
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The process of chromosome
duplication followed by chromosome
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separation, chromatid separation
really, which allows daughter cells,
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during this process of mitosis to
faithfully inherit all of the DNA.
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And this process continues through
development, and it also continues
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in adults. It's not over when the
baby is born. There's a lot of
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mitosis that takes place in many of
your organs, and your intestines for
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example, your blood.
Not in all of your organs.
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In the brain, for example, there is
relatively little mitosis.
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Once the brain forms, cells don't
divide anymore.
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It's true of the cardiac muscle as
well in the heart.
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But many tissues do continue to
divide. So this is happening inside
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of you as we speak.
Now, what I want to turn to for
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today, though,
is to talk about this process.
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How do you go back to the beginning?
How do you generate haploid cells
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from diploid cells?
And this occurs through a related
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process called meiosis.
Now, meiosis takes place at
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different times,
depending on whether you're male or
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female. As I mentioned last time in
males meiosis takes place
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in puberty.
So diploid germ cells are set aside
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in embryogenesis,
but they don't undergo this process
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of meiosis until males reach puberty
at which point it happens very
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abundantly, as I said,
about a million or a couple of
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million meiotic cells produced per
hour. In females meiosis actually
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begins in embryogenesis,
quite amazing. The cells are set
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aside and they start to undergo
meiosis. And they don't make it all
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the way through.
You don't make haploid germ cells
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in embryogenesis.
They get stuck partway through.
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Then in puberty,
during ovulation a subset of those
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cells continue through the
subsequent stages of meiosis.
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Then they get stuck again. And
only at fertilization does meiosis
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get completed,
a haploid cell is produced which is
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then fused with the sperm cell,
a diploid cell is generated and
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development proceeds.
So what is meiosis and how does it
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compare to mitosis?
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Well, they're related in the sense
that both of them follow S phase.
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If you remember during S phase
chromosomes get duplicated.
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And during chromosome duplication
two chromatids are produced.
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You're not going to start singing,
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are you? No. Just kidding. When a
chromosome gets produced two
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chromatids are generated and then
they stick together at the
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centromere. Remember that?
Now, in mitosis, as we talked about
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before, this is followed by one
round of chromatid separation.
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That's all it is.
The chromosomes with their two
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chromatids line up along the
metaphase plate,
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and they get pulled apart during
anaphase and telophase.
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And importantly the homologs don't
pay any attention to one another.
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The maternal copy of chromosome one
and the paternal copy of chromosome
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two ignore each other in mitosis.
They could be anywhere along that
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metaphase plate.
It doesn't matter.
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They ignore one another.
OK? That's relevant because it's
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different in meiosis.
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In meiosis, also following one round
of chromosome duplication and the
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generation of two chromatids per
chromosome, this involves one round
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of chromosome separation.
And when I say chromosome here,
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I'm talking about the chromosome
which is at this point composed of
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two chromatids.
In this case the homologs do pay
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attention to one another and they
separate from one another.
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Followed by one round of chromatid
separation.
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Which is very similar to what
happens over here.
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OK? So the way you go from having
46 chromosomes to 23 chromosomes is
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you go through one round of
duplication but two rounds of
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separation. The way you maintain 46
chromosomes in mitosis is to go
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through one round of duplication and
only one round of separation.
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Make sense? OK. So let's look at
that in detail.
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And I won't draw these pictures on
the board for you because,
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actually, I want to review it in
writing one more time.
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I'm not going to draw the details
of meiosis for you on the board
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because it just takes too long,
and the book does a perfectly good
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job diagramming it for you.
And I'll show you those diagrams in
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a second. The terms that we use in
meiosis are the same terms that we
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use in mitosis.
But there are two rounds,
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as I mentioned, and they're broken
down by meiosis one and meiosis two.
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In meiosis one you have a prophase,
and it's called prophase one.
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You have a prometaphase,
prometaphase one.
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A metaphase where the chromosomes
align on the metaphase plate.
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And anaphase where the chromosomes
get pulled apart by the mitotic
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spindle, actually,
in this case the meiotic spindle.
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And finally a telophase where they
get pulled all the way to the poles.
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And then there's a second round,
meiosis two, where the same terms
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are applied but they're
denoted with twos.
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So there's a second phase of
prophase, of prometaphase,
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metaphase, anaphase and telophase.
And this is where the chromatids
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align on the chromosome plate and
get separated.
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OK? Now, there's a very key event,
and I'm going to draw it for you
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because it's so important,
that takes place during prophase of
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meiosis one. And this is the
distinguishing feature between
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meiosis and mitosis.
As I said in mitosis,
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the homologs, the two copies of
chromosome one,
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the maternal one and the paternal
one for example ignore one another.
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That's not true in mitosis, in
meiosis. In meiosis they bind to
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one another. So imagine again our,
I think I called this the paternal
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copy of chromosome whatever,
one previously. It had four genes
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that we showed previously.
It's represented as two chromatids,
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which are associated at the
centromere. And then you have
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another homolog,
the maternal homolog.
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It likewise has the same four genes.
It's been duplicated so it's
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represented as two chromatids held
together at the centromere.
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In prophase of meiosis one,
these homologs pair.
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And it's the pairing of the homologs
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that allows the two to separate
faithfully during meiosis one.
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They also interact in an
interesting way that we'll come to
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in a moment. They don't just pair.
They actually undergo interactions
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between each other which allows them
to exchange sequences which is
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critical for generating diversity
amongst our chromosomes.
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So now I'm going to take you through
these steps as shown in your book.
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This is figure 9.14 in your book.
We're going to go through meiosis
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one first. So in prophase of
meiosis one, we've undergone
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chromosome duplication already into
the chromatids.
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Now the chromosomes are going to
condense, and you can see that here.
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And importantly during prophase the
two chromosomes,
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represented by the four chromatids
align.
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In this picture you're seeing two
different chromosomes,
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a larger one and a smaller one.
And you can see that the maternal
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and paternal copies have paired with
one another. OK?
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So we get pairs of homologs.
Still later in prophase they
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interact in this other fashion that
I just alluded to where they
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actually exchange information.
They exchange genetic information.
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And we'll go through this in a
little bit more detail in a bit.
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At metaphase of meiosis one,
metaphase one,
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the homologs line up on the
metaphase plate with one homolog to
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the left, the other homolog to the
right, one homolog to the left,
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the other homolog to the right.
This is one of two possible
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configurations that could take place
here. In fact,
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in another cell undergoing metaphase
one, the red one might be on the
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left, the blue one on the right,
but here the blue one might be one
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the left, the red one might
be on the right.
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And that's important.
You'll see why later.
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So the homologs line up along the
metaphase plate.
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And now during anaphase one the
homologs separate from one another.
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The two chromatids of each homolog
separate from one another,
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and that's completed in telophase.
Now, without an intervening round
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of DNA duplication,
this does not involve another round
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of S phase, we go straight into
meiosis two where again the DNA
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condenses, a mitotic
spindle is built.
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And now, just like in mitosis,
the two chromatids line up along the
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metaphase plate with one chromatid
to the top, the other chromatid to
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the bottom, one chromatid to the top,
the other to the bottom.
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And then they get separated during
anaphase and telophase
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of meiosis two.
The end product of that are germ
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cells which are now haploid.
They have only one copy of each of
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the chromosomes,
one of the big ones and one of the
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small ones. So that's how you go
from a cell that has two copies of
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each to a cell that has one copy of
each. One round of DNA duplication,
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two rounds of chromosome or
chromatid separation.
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Now, importantly you'll see if you
can squint a little bit,
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that the chromosomes that come out
of this process of meiosis don't
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always look, in fact,
never look like the chromosomes that
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come in. They undergo this process
of exchange of genetic information.
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This process of exchange of genetic
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information is called meiotic
recombination.
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And I'll show it to you in a figure
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in a second. But it generates first
recombinant chromatids,
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chromatids that look different from
the chromatids that were generated
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initially. That's why they're
called recombinant.
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And these, when resolved during
meiosis two, generate recombinant
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chromosomes.
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So, again, if we imagine our two
homologs of a given chromosome with
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the four genes that we talked about
previously lined up along the length
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of those chromosomes,
during this process of meiotic
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recombination --
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-- new versions of the chromosomes
are produced. We can generate a
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chromosome which has a little bit of
Dad's DNA and a little
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bit of Mom's DNA.
A chromosome that was generated
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specifically in you.
It's your unique contribution to
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the offspring.
It's not solely what you got from
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Mom or Dad. It's your unique
version of that chromosome that's a
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hybrid between what you got from Mom
and what you go from Dad.
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And this process of meiotic
recombination allows for the
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increased diversity in the
population. You don't just pass on
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what you inherited.
You actually mix it up a little bit
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and pass on new combinations of the
alleles that you got from your
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parents. So this is shown in
greater detail here.
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And, again, gone through in much
detail in your book.
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This is figure 9.
6. Here we are in prophase of
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meiosis one. The homologs have
paired and they've undergone a
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genetic interaction.
The DNA of the paternal and the
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maternal chromatids have literally
exchanged information.
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That's what this crossover is
called. It's referred to as a
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chiasma. And you can literally see
it in the electron microscope.
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When this gets resolved,
like a cut and paste reaction.
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When this gets resolved, you
generate new chromatids.
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This chromatid has mostly red
sequence but a little bit of blue
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sequence. And this chromatid has
mostly blue sequence and a little
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bit of red sequence.
If you think about that with
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respect to genes in our analogy over
here, the red chromosome had alleles
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that we'll refer to as the white
versions of gene one,
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two, three and four.
The blue homolog had the black
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versions of genes one,
two, three and four. The products
250
00:20:36 --> 00:20:41
of this reaction include chromatids
that look just like the parental
251
00:20:41 --> 00:20:45
ones, a red chromosome with white
one, two, three and four,
252
00:20:45 --> 00:20:49
a black chromosome with, a blue
chromosome with black one,
253
00:20:49 --> 00:20:54
two, three and four, but also new
chromatids that didn't exist
254
00:20:54 --> 00:20:58
previously, a red one with white one,
two, three but a black four,
255
00:20:58 --> 00:21:03
and a blue one with black one two
and three and a white four.
256
00:21:03 --> 00:21:13
New versions of the chromosomes.
And this is important. It's
257
00:21:13 --> 00:21:23
important for you to understand how
genetics works,
258
00:21:23 --> 00:21:34
but it's also important for the
species to generate increased
259
00:21:34 --> 00:21:44
diversity which when acted upon by
natural selection forces might
260
00:21:44 --> 00:21:54
select out more fit organisms.
So that is mitosis, meiosis and
261
00:21:54 --> 00:22:02
meiotic recombination.
And we'll be drawing on your
262
00:22:02 --> 00:22:06
knowledge of meiosis and mitotic
recombination to understand the
263
00:22:06 --> 00:22:11
principles of genetics.
And that will be the next topic
264
00:22:11 --> 00:22:16
that we turn to.
Before I do so I want to just
265
00:22:16 --> 00:22:20
mention that this process of meiosis
doesn't always work perfectly.
266
00:22:20 --> 00:22:25
No biological system works
perfectly. There's always some
267
00:22:25 --> 00:22:29
error rate. And that's true in
meiosis in germ cell development,
268
00:22:29 --> 00:22:34
although it's supposed to be the
case that during metaphase one the
269
00:22:34 --> 00:22:39
homologs, which are paired together,
separate to the two daughters.
270
00:22:39 --> 00:22:44
That doesn't always happen.
Occasionally errors take place such
271
00:22:44 --> 00:22:50
that one daughter gets nothing and
the other daughter gets both copies.
272
00:22:50 --> 00:22:55
This is a problem because now when
these guys go through meiosis two
273
00:22:55 --> 00:23:01
they have two copies of a given
chromosome instead of just one.
274
00:23:01 --> 00:23:05
And if those germ cells,
in this case they're eggs,
275
00:23:05 --> 00:23:10
and that's usually the problem in
humans where it occurs,
276
00:23:10 --> 00:23:14
if those eggs get fused by a sperm
that carries its own copy of that
277
00:23:14 --> 00:23:19
chromosome the zygote,
instead of having just two copies,
278
00:23:19 --> 00:23:24
has three. And this is a situation
called trisomy,
279
00:23:24 --> 00:23:29
an extra copy of a given chromosome.
The opposite can happen as well.
280
00:23:29 --> 00:23:32
This guy, this egg,
if it were to be fertilized would
281
00:23:32 --> 00:23:36
have only a single copy of that
given chromosome.
282
00:23:36 --> 00:23:40
That would be monosomy,
single instead of two. This does
283
00:23:40 --> 00:23:44
happen during human development.
In the vast majority of cases when
284
00:23:44 --> 00:23:48
it happens it's incompatible with
early development and the fetus
285
00:23:48 --> 00:23:52
aborts. There are a few examples
where the fetus can actually make it
286
00:23:52 --> 00:23:56
quite late in gestation and even be
born but then doesn't
287
00:23:56 --> 00:24:00
thrive thereafter.
But there's one example where the
288
00:24:00 --> 00:24:04
fetus comes to term,
is born and the individual can
289
00:24:04 --> 00:24:08
survive, and that's chromosome 21
trisomy or Down syndrome.
290
00:24:08 --> 00:24:12
This happens when during meiosis
one nondisjunction takes place such
291
00:24:12 --> 00:24:17
that two copies of chromosome 21
wind up in an egg.
292
00:24:17 --> 00:24:21
Sperm comes along and delivers
another copy of chromosome 21.
293
00:24:21 --> 00:24:25
Now that zygote is trisomic for
chromosome 21.
294
00:24:25 --> 00:24:30
And you can see the karyotype here.
295
00:24:30 --> 00:24:34
Everything else is diploid but this
has three copies of chromosome 21.
296
00:24:34 --> 00:24:38
And this leads to a characteristic
defect in development which results
297
00:24:38 --> 00:24:42
in an individual with very
characteristic features.
298
00:24:42 --> 00:24:46
And this extra copy of 21 is
responsible for this too much gene
299
00:24:46 --> 00:24:50
product from the genes on chromosome
21. And I don't remember how many
300
00:24:50 --> 00:24:54
there are. There are 600 or so
genes on chromosome 21.
301
00:24:54 --> 00:24:58
Having too much of some or some of
them, some combination of them,
302
00:24:58 --> 00:25:02
too much product from those leads to
defects in development,
303
00:25:02 --> 00:25:06
which results in this very clear
phenotype.
304
00:25:06 --> 00:25:10
So meiosis usually insures that you
get just one copy of each
305
00:25:10 --> 00:25:15
chromosomes, but it doesn't always
work. And this can be one
306
00:25:15 --> 00:25:19
consequence. OK.
Let's take a breath and change
307
00:25:19 --> 00:25:24
gears. So we're going to move now
from mitosis and meiosis to genetics
308
00:25:24 --> 00:25:29
and genetic principles,
and specifically Mendelian genetics.
309
00:25:29 --> 00:25:32
So what we want to now begin to
understand is besides the mechanics,
310
00:25:32 --> 00:25:36
what are the consequences? What
happens when you inherit alleles of
311
00:25:36 --> 00:25:40
given genes? What is the effect of
the inheritance of those alleles and
312
00:25:40 --> 00:25:44
what are the principles that guide
that? Why is it that when a child
313
00:25:44 --> 00:25:47
is born of two blue-eyed people that
child has blue eyes?
314
00:25:47 --> 00:25:51
When a child is born of a
brown-eyed person and blue-eyes
315
00:25:51 --> 00:25:55
person that child is likely to have
brown eyes, but not always.
316
00:25:55 --> 00:25:59
Maybe they'd have blue eyes.
What governs the presentation of a
317
00:25:59 --> 00:26:03
given trait based on the genes that
that individual inherits?
318
00:26:03 --> 00:26:06
Or more precisely the alleles of the
genes that individual inherits.
319
00:26:06 --> 00:26:10
You might actually be able to see
that the murderer can be seen in the
320
00:26:10 --> 00:26:13
reflection of this person's eye
right here. You've got to look
321
00:26:13 --> 00:26:17
carefully. We can learn a lot about
this by looking at inheritance
322
00:26:17 --> 00:26:20
patterns in humans.
And that's increasingly powerful
323
00:26:20 --> 00:26:24
given our knowledge of the human
genome today. There's a lot more we
324
00:26:24 --> 00:26:28
can do in human genetics today than
we could have done ten years ago.
325
00:26:28 --> 00:26:32
But the field has actually been
brought along to this point using
326
00:26:32 --> 00:26:36
genetics in model organisms.
And this is a fly, a fruit fly,
327
00:26:36 --> 00:26:40
drosophila melanogaster, a major
genetic organism in biology.
328
00:26:40 --> 00:26:44
And you can see again that you can
isolate flies with different colored
329
00:26:44 --> 00:26:48
eyes, red eyes,
white eyes, dark eyes.
330
00:26:48 --> 00:26:52
And you can understand the
principles of genetics by crossing,
331
00:26:52 --> 00:26:56
mating flies together and looking at
what happens in the eye color of the
332
00:26:56 --> 00:27:00
offspring of flies that start off
with a given eye color.
333
00:27:00 --> 00:27:05
Now, the principles that guide us in
our thinking about genetics derive
334
00:27:05 --> 00:27:10
from this individual here whose name
is Gregor Mendel.
335
00:27:10 --> 00:27:15
And the principles that he laid
down are referred to as Mendelian
336
00:27:15 --> 00:27:21
inheritance. And he,
through work that I'll briefly
337
00:27:21 --> 00:27:26
summarize, developed certain laws of
inheritance which turned
338
00:27:26 --> 00:27:31
out to be true.
And we use them even now as we think
339
00:27:31 --> 00:27:35
about how genes and alleles get
passed on through the generations.
340
00:27:35 --> 00:27:40
Mendel lived in the 1800s. He did
his work around 1850s,
341
00:27:40 --> 00:27:44
1860s. His organism was the pea
plant and the peas that give rise to
342
00:27:44 --> 00:27:48
the pea plant.
And he spent a lot of time
343
00:27:48 --> 00:27:53
observing peas and pea plants,
breading or crossing pea plants
344
00:27:53 --> 00:27:57
together, looking at the products of
those crosses and coming
345
00:27:57 --> 00:28:01
up with these theories.
He published a paper on that work in,
346
00:28:01 --> 00:28:04
I think, 1865,
and it was roundly ignored.
347
00:28:04 --> 00:28:07
Nobody paid any attention. I
should have mentioned that he was an
348
00:28:07 --> 00:28:10
Austrian monk.
He lived up in the hills in Austria
349
00:28:10 --> 00:28:13
and did his work largely in
seclusion. But he did,
350
00:28:13 --> 00:28:16
in fact, publish a paper in 1865
reporting his principles of
351
00:28:16 --> 00:28:19
inheritance. And he was largely
ignored until about 50 years later
352
00:28:19 --> 00:28:22
when other workers started to do
similar things and basically
353
00:28:22 --> 00:28:25
rediscovered these principles that
he had laid down so many years
354
00:28:25 --> 00:28:28
before. And we now give him credit
for coming up with these principles
355
00:28:28 --> 00:28:32
in the first place.
So Mendel focused on traits,
356
00:28:32 --> 00:28:37
traits that he could observe in the
pea plant or in the peas themselves.
357
00:28:37 --> 00:28:42
And this is an example of the
traits that he might look at.
358
00:28:42 --> 00:28:47
You may know that peas can come in
different shapes and textures.
359
00:28:47 --> 00:28:52
There are peas that are smooth.
There are peas that are wrinkled.
360
00:28:52 --> 00:28:57
And Mendel wondered what controlled
whether a pea was smooth
361
00:28:57 --> 00:29:01
or wrinkled.
If you crossed a pea plant that
362
00:29:01 --> 00:29:04
would have produced smooth peas with
a pea plant that would have produced
363
00:29:04 --> 00:29:08
wrinkled peas,
do you get smooth or wrinkled peas?
364
00:29:08 --> 00:29:11
The principles that guided the
thinking previously were that this
365
00:29:11 --> 00:29:14
was some sort of a mixture.
If you had one type and another
366
00:29:14 --> 00:29:18
type, the offspring would have some
intermediate type.
367
00:29:18 --> 00:29:21
There was sort of a mixing of
genetic information,
368
00:29:21 --> 00:29:24
not really referred to as genetic
information at the time,
369
00:29:24 --> 00:29:28
but heritable information.
And Mendel wondered whether that
370
00:29:28 --> 00:29:32
was true.
And then he actually determined that
371
00:29:32 --> 00:29:36
for most things that was not true.
One type determined the appearance
372
00:29:36 --> 00:29:41
of the trait in the offspring.
In order for you to understand what
373
00:29:41 --> 00:29:46
Mendel did, you have to understand a
little bit about pea plants and peas.
374
00:29:46 --> 00:29:50
One thing you need to know is that
peas are the embryo.
375
00:29:50 --> 00:29:55
They are the product of
fertilization.
376
00:29:55 --> 00:30:00
And when you plant a pea it will
develop into a pea plant.
377
00:30:00 --> 00:30:04
And you can score traits either in
the pea itself,
378
00:30:04 --> 00:30:09
in the embryo, smooth,
wrinkled, dark green, light green,
379
00:30:09 --> 00:30:13
or in the pea plant that results
from that pea.
380
00:30:13 --> 00:30:18
Does it have red flowers or white
flowers or pink flowers?
381
00:30:18 --> 00:30:23
And importantly you also can
fertilize one pea plant from another.
382
00:30:23 --> 00:30:27
You can take the germ cells,
the male germ cells or male gametes
383
00:30:27 --> 00:30:32
from one pea plant,
and purposely fertilize a female
384
00:30:32 --> 00:30:37
gamete of another pea plant.
You can carry out these very precise
385
00:30:37 --> 00:30:41
crosses. Or you can do it within
the same pea plant.
386
00:30:41 --> 00:30:45
You can take the male germ cells or
gametes and fertilize the female
387
00:30:45 --> 00:30:49
gametes from that same plant,
so-called self-pollination. OK?
388
00:30:49 --> 00:30:53
And through this methodology,
Mendel was able to very precisely
389
00:30:53 --> 00:30:57
control what the pea plants looked
like and how he could
390
00:30:57 --> 00:31:07
manipulate them.
391
00:31:07 --> 00:31:12
Through this process of
self-pollination,
392
00:31:12 --> 00:31:18
Mendel was able to generate what he
called “pure breading strains”.
393
00:31:18 --> 00:31:23
That is they always produced the
same trait. If you cross them
394
00:31:23 --> 00:31:29
together, if you fertilize,
self-fertilize them, you always got
395
00:31:29 --> 00:31:34
the same result,
always smooth peas or always
396
00:31:34 --> 00:31:40
wrinkled peas or always red flowers
or always white flowers.
397
00:31:40 --> 00:31:43
They were pure.
There wasn't any heterogeneity.
398
00:31:43 --> 00:31:46
And we're going to refer to these
in our discussion as
399
00:31:46 --> 00:31:55
parental strains.
400
00:31:55 --> 00:32:02
And kind of an example of a question
that Mendel would want to know is if
401
00:32:02 --> 00:32:09
you took two parental strains and he
crossed pollinated from one that
402
00:32:09 --> 00:32:16
always produced smooth peas,
and here we're talking about this X
403
00:32:16 --> 00:32:23
represents crossing fertilization,
with a plant that always produced
404
00:32:23 --> 00:32:30
wrinkled peas, what did the
offspring look like?
405
00:32:30 --> 00:32:36
What did the pea look like in the
product of that cross?
406
00:32:36 --> 00:32:43
These parental strains are referred
to as P. The product of a cross
407
00:32:43 --> 00:32:49
between two parental strains is
called the F1 or first
408
00:32:49 --> 00:33:02
filial generation.
409
00:33:02 --> 00:33:08
And the question Mendel wanted to
know was what is the P type in the
410
00:33:08 --> 00:33:14
F1? The answer turns out,
for this particular example,
411
00:33:14 --> 00:33:20
to be smooth peas. And the question
is why? Based on this kind of
412
00:33:20 --> 00:33:27
observation, Mendel generated
a hypothesis.
413
00:33:27 --> 00:33:47
He suggested that traits in the peas,
414
00:33:47 --> 00:34:02
as well as in the subsequent plants,
arise from the inheritance of two
415
00:34:02 --> 00:34:12
units.
These units we would think of now as
416
00:34:12 --> 00:34:18
two alleles of a given gene.
He wasn't thinking of genes or
417
00:34:18 --> 00:34:23
alleles. He was just thinking of
what number of things was
418
00:34:23 --> 00:34:29
contributing to this particular
trait. And that these two units
419
00:34:29 --> 00:34:35
were delivered to the embryo
from each parent.
420
00:34:35 --> 00:34:49
And importantly the parent then has
421
00:34:49 --> 00:35:03
two units so that each parent
delivers one of its two alleles to
422
00:35:03 --> 00:35:17
the offspring via the production
of germ cells.
423
00:35:17 --> 00:35:20
So everybody has two.
They pass on one of those two to
424
00:35:20 --> 00:35:24
their germ cells.
And then the embryo is the product
425
00:35:24 --> 00:35:28
of two germ cells coming together
generating, once again,
426
00:35:28 --> 00:35:36
something with two units. OK?
427
00:35:36 --> 00:35:41
So let's think about this with
respect to pea plants,
428
00:35:41 --> 00:35:46
and specifically the shape of the
peas. So the trait that we're
429
00:35:46 --> 00:35:52
interested in is smooth
or wrinkled peas.
430
00:35:52 --> 00:36:03
The gene, in our language,
431
00:36:03 --> 00:36:10
is the S gene. It is going to
determine whether or not the pea is
432
00:36:10 --> 00:36:18
smooth or wrinkled.
And this gene comes in two
433
00:36:18 --> 00:36:25
varieties, two alleles.
One is called big S and the other
434
00:36:25 --> 00:36:32
is called little S.
In the cross that was done,
435
00:36:32 --> 00:36:39
the plant that always generated
smooth peas, which was a pure
436
00:36:39 --> 00:36:46
breeding strain,
had the same allele in both copies.
437
00:36:46 --> 00:36:53
It was diploid for the big S allele.
Both of its chromosomes that carry
438
00:36:53 --> 00:37:00
the S gene had the big S
version of that gene.
439
00:37:00 --> 00:37:07
And the wrinkled plant,
the plant that produced wrinkled
440
00:37:07 --> 00:37:15
peas carried two copies of the other
allele, little S.
441
00:37:15 --> 00:37:23
These plants produced smooth peas.
These plants produced wrinkled peas.
442
00:37:23 --> 00:37:31
When these plants produced germ
cells, what allele of this gene gets
443
00:37:31 --> 00:37:38
put into those germ cells?
Big S. It's the only one there.
444
00:37:38 --> 00:37:45
So when these plants undergo
meiosis they're going to produce
445
00:37:45 --> 00:37:53
germ cells that carry in them the
big S gene. When these plants
446
00:37:53 --> 00:38:00
undergo meiosis what allele of the S
gene did they put in their
447
00:38:00 --> 00:38:07
germ cells? Little S.
These haploid germ cells have the
448
00:38:07 --> 00:38:13
little S gene.
Now, when I cross,
449
00:38:13 --> 00:38:19
I take one of these germ cells,
mix it with one of those germ cells
450
00:38:19 --> 00:38:25
in the cross, what gene,
what genes, what alleles does that
451
00:38:25 --> 00:38:33
offspring inherit?
It gets a big S from here and it
452
00:38:33 --> 00:38:41
gets a little S from here.
OK? And what do I observe when I
453
00:38:41 --> 00:38:49
make an organism,
a pea, that looks like that?
454
00:38:49 --> 00:38:58
What does the trait look like?
It's smooth. OK.
455
00:38:58 --> 00:39:04
Now, I've drawn details without
given you some of the relevant
456
00:39:04 --> 00:39:11
nomenclature. The parental strain
over here had big S,
457
00:39:11 --> 00:39:18
big S. We call that the genotype.
A genotype refers to which alleles
458
00:39:18 --> 00:39:25
you have; your genotype.
The genotype of the other parental
459
00:39:25 --> 00:39:32
strain is S, S.
And the genotype of the offspring is
460
00:39:32 --> 00:39:39
big S, little S.
That's the genotype.
461
00:39:39 --> 00:39:46
If you have two of the same alleles
it is called being homozygous.
462
00:39:46 --> 00:39:53
This also has two of the same
alleles so it is also homozygous for
463
00:39:53 --> 00:40:00
the other allele but
still homozygous.
464
00:40:00 --> 00:40:08
If you have one allele of one type
and the other allele you are called
465
00:40:08 --> 00:40:16
heterozygous. OK.
So that's some relevant terminology.
466
00:40:16 --> 00:40:24
Now, in contrast to the genotype,
we also refer to the phenotype. And
467
00:40:24 --> 00:40:33
the phenotype refers to the
manifestation of the trait.
468
00:40:33 --> 00:40:40
What you look like.
Regardless of what the gene
469
00:40:40 --> 00:40:47
combination or allele combination
you have, what do you actually look
470
00:40:47 --> 00:40:54
like? So what is the phenotype of
these peas? Smooth.
471
00:40:54 --> 00:41:02
And what is the phenotype of these
peas? They're wrinkled.
472
00:41:02 --> 00:41:09
And what is the phenotype of these
peas? They're smooth.
473
00:41:09 --> 00:41:17
Based on the patterns that he
observed when he crossed two pure
474
00:41:17 --> 00:41:25
breading strains and generated
offspring that resembled one of the
475
00:41:25 --> 00:41:33
two pure breading strains,
Mendel suggested that in this
476
00:41:33 --> 00:41:41
particular cross the big S unit,
we would call it allele, is dominant
477
00:41:41 --> 00:41:48
over little S.
If you have a big S allele,
478
00:41:48 --> 00:41:54
regardless of what you have as the
other allele, you're going to have
479
00:41:54 --> 00:42:00
the big S trait which is smooth.
Big S is dominant over little S.
480
00:42:00 --> 00:42:06
And a related term,
little S is recessive to big S.
481
00:42:06 --> 00:42:13
The only way you observe the
phenotype associated with the little
482
00:42:13 --> 00:42:20
S allele is if you're homozygous for
the little S. If you're
483
00:42:20 --> 00:42:27
heterozygous, as in the case in the
middle, you manifest the phenotype
484
00:42:27 --> 00:42:34
associated with the other allele.
Dominance and recessive.
485
00:42:34 --> 00:42:41
OK. Now, based on these ideas of
inheritance of a single unit or
486
00:42:41 --> 00:42:47
allele from the parents who carry
two and the concept of dominance and
487
00:42:47 --> 00:42:54
recessiveness,
you can come up with methods for
488
00:42:54 --> 00:43:01
determining the frequency of
observing genotypes and phenotypes
489
00:43:01 --> 00:43:08
in crosses such as this.
So I want to consider now not a
490
00:43:08 --> 00:43:16
parental cross giving rise to an F1,
but rather a cross between F1. We
491
00:43:16 --> 00:43:24
call this a hybrid cross.
And because we're focusing on one
492
00:43:24 --> 00:43:32
gene we actually call it
a monohybrid cross.
493
00:43:32 --> 00:43:44
The genotypes of the plants that
494
00:43:44 --> 00:43:49
we're going to start with in the F1
generation are as we described
495
00:43:49 --> 00:43:54
previously. They're heterozygous
big S, little S.
496
00:43:54 --> 00:44:00
And that's true for one plant and
the other.
497
00:44:00 --> 00:44:07
They're both F1s.
They're both heterozygous for big S,
498
00:44:07 --> 00:44:14
little S. If we now cross these
together, we can think about what
499
00:44:14 --> 00:44:22
germ cells these plants produce.
So what are the four products of
500
00:44:22 --> 00:44:29
meiosis look like from this?
What do you get in the meiotic
501
00:44:29 --> 00:44:37
products starting with
these genotypes?
502
00:44:37 --> 00:44:43
Two of them get big S,
two of them get little S.
503
00:44:43 --> 00:44:49
And likewise over here. Two of the
germ cells get big S,
504
00:44:49 --> 00:44:55
two of the germ cells get little S.
Therefore, half of the germ cells
505
00:44:55 --> 00:45:01
are big S, half are little S.
If I think about the frequency of
506
00:45:01 --> 00:45:06
big S germ cells it's one-half.
If I think of the frequency of
507
00:45:06 --> 00:45:12
little S germ cells it's one-half.
And likewise over here, half of the
508
00:45:12 --> 00:45:17
germ cells are big S,
half of the germ cells are little S.
509
00:45:17 --> 00:45:23
Based on this idea I can generate a
grid --
510
00:45:23 --> 00:45:32
-- which will allow me to calculate
511
00:45:32 --> 00:45:37
the frequency of offspring that have
four different combinations.
512
00:45:37 --> 00:45:43
If this germ cell combines with
this germ cell I'll be big S,
513
00:45:43 --> 00:45:48
big S. If this germ cell combines
with this germ cell the genotype
514
00:45:48 --> 00:45:53
will be big S,
little S. If this germ cell
515
00:45:53 --> 00:45:59
combines with this germ cell the
genotype will be little
516
00:45:59 --> 00:46:04
S, little S.
And if this germ cell combines with
517
00:46:04 --> 00:46:08
this germ cell the genotype will be
big S, little S.
518
00:46:08 --> 00:46:13
This box, which is a useful device
for calculating the frequency of the
519
00:46:13 --> 00:46:17
genotypes that you observe in
different crosses,
520
00:46:17 --> 00:46:31
is referred to as a Punnett Square.
521
00:46:31 --> 00:46:36
So let's think about what the
numbers would be in such a
522
00:46:36 --> 00:46:42
monohybrid cross.
There are three possible genotypes.
523
00:46:42 --> 00:46:48
If you look at the Punnett Square
there are three possible genotypes.
524
00:46:48 --> 00:46:54
You can be homozygous big S, big S.
You can be homozygous little S,
525
00:46:54 --> 00:47:00
little S. Or you can be
heterozygous big S, little S.
526
00:47:00 --> 00:47:07
Those are the three possible
genotypes from a cross such as this.
527
00:47:07 --> 00:47:15
The ratio of those genotypes, if
you look in the Punnett Square is
528
00:47:15 --> 00:47:23
there is one of these for every one
of these and two of these.
529
00:47:23 --> 00:47:31
The ratio of the three genotypes is
1:2:1 as determined by this
530
00:47:31 --> 00:47:37
kind of calculation.
The reason that you get that ratio
531
00:47:37 --> 00:47:43
is that a quarter of these,
sorry. Half of these germ cells are
532
00:47:43 --> 00:47:48
big S, half of these germ cells are
big S, so a quarter plus,
533
00:47:48 --> 00:47:54
times a quarter, sorry, sorry,
sorry, I said that wrong. Half of
534
00:47:54 --> 00:48:00
these germ cells are big S.
Half of those germ cells are big S.
535
00:48:00 --> 00:48:05
So a half times a half is a quarter.
Half of these germ cells are little
536
00:48:05 --> 00:48:10
S. Half of these germ cells are
little S. The probability of
537
00:48:10 --> 00:48:15
getting one of each is a half times
a half or a quarter.
538
00:48:15 --> 00:48:20
For big S, little S you can do it
two different ways,
539
00:48:20 --> 00:48:26
so it's a quarter plus a quarter or
a half.
540
00:48:26 --> 00:48:35
The ratio of these fractions is
541
00:48:35 --> 00:48:42
1:2:1 which determines the products
that you see. Now,
542
00:48:42 --> 00:48:49
before I let you go,
I want you think about the
543
00:48:49 --> 00:48:56
phenotypes. The phenotypes that you
observe in these peas is smooth,
544
00:48:56 --> 00:49:03
in these peas is smooth and in these
peas wrinkled.
545
00:49:03 --> 00:49:07
So the ratio of phenotypes is 2:1.
You'll need to think about why
546
00:49:07 --> 00:49:12
those, sorry, sorry,
two plus one is three,
547
00:49:12 --> 00:49:16
3:1. You'll need to think about why
those principles give rise to these
548
00:49:16 --> 00:49:19
calculations and these frequencies.